Textbook Question
(c) Do catalysts affect the overall enthalpy change for a reaction, the activation energy, or both?
Ch.14 - Chemical Kinetics
Chapter 14, Problem 78
In solution, chemical species as simple as H+ and OH- can serve as catalysts for reactions. Imagine you could measure the [H+] of a solution containing an acid-catalyzed reaction as it occurs. Assume the reactants and products themselves are neither acids nor bases. Sketch the [H+] concentration profile you would measure as a function of time for the reaction, assuming t = 0 is when you add a drop of acid to the reaction.
Verified step by step guidance1
Step 1: Understand that the addition of an acid to the reaction at t = 0 will increase the concentration of hydrogen ions, [H+], in the solution.
Step 2: Recognize that as the acid-catalyzed reaction proceeds, the [H+] may initially remain relatively constant if the acid is in excess and the reaction is slow compared to the rate of acid consumption.
Step 3: Consider that as the reaction progresses, the [H+] may start to decrease if the acid is consumed faster than it is replenished, or if the reaction reaches completion.
Step 4: Sketch a graph with time on the x-axis and [H+] on the y-axis, starting with a high [H+] at t = 0, then showing a plateau if the acid is in excess, followed by a potential decrease as the reaction consumes the acid.
Step 5: Label the graph to indicate the initial increase in [H+], the plateau phase, and any decrease in [H+] as the reaction proceeds and the acid is consumed.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acid-Base Chemistry
Acid-base chemistry involves the study of proton donors (acids) and proton acceptors (bases). In aqueous solutions, acids release H+ ions, while bases produce OH- ions. Understanding the behavior of these ions is crucial for predicting how they influence reaction rates and mechanisms, particularly in acid-catalyzed reactions.
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Arrhenius Acids and Bases
Catalysis
Catalysis refers to the process by which the rate of a chemical reaction is increased by a substance called a catalyst, which is not consumed in the reaction. In this context, H+ ions act as a catalyst by providing protons that facilitate the reaction without being permanently altered, thus affecting the concentration profile of H+ over time.
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01:59Catalyzed vs. Uncatalyzed Reactions
Concentration Profiles
A concentration profile illustrates how the concentration of a particular species changes over time during a reaction. In this scenario, measuring the [H+] concentration as a function of time will show an initial increase due to the added acid, followed by a potential decrease as the reaction proceeds, reflecting the dynamic nature of the acid-catalyzed process.
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Related Practice
Textbook Question
(a) Most commercial heterogeneous catalysts are extremely finely divided solid materials. Why is particle size important?
Textbook Question
The addition of NO accelerates the decomposition of N2O, possibly by the following mechanism: NO1g2 + N2O1g2¡N21g2 + NO21g2 2 NO21g2¡2 NO1g2 + O21g2 (b) Is NO serving as a catalyst or an intermediate in this reaction?